For the production of inosine and guanosine by fermentation, there have been known methods utilizing adenine auxotrophic strains or such strains further imparted with drug resistance against various drugs such as purine analogues, which strains belong to the genus Bacillus (Japanese Patent Publication Nos. 38-23039 (1963), 54-17033 (1979), 55-2956 (1980), and 55-45199 (1980), Japanese Patent Application Laid-Open No. 56-162998 (1981), Japanese Patent Publication Nos. 57-14160 (1982) and 57-41915 (1982), and Japanese Patent Application Laid-Open No. 59-42895 (1984)), or the genus Brevibacterium (Japanese Patent Publication Nos. 51-5075 (1976) and 58-17592 (1972), and Agric. Biol. Chem., 42, 399 (1978)) and the like.
Conventional acquisition of such mutant strains comprises subjecting microorganisms to a mutagenesis treatment such as UV irradiation and nitrosoguanidine (N-methyl-N′-nitro-N-nitrosoguanidine) treatment and selecting a desired strain by using a suitable selection medium. On the other hand, breeding of such mutant strains by the use of genetic engineering techniques have also been practiced for strains belonging to the genus Bacillus (Japanese Patent Application Laid-Open Nos. 58-158197 (1983), 58-175493 (1983), 59-28470 (1984), 60-156388 (1985), 1-27477 (1989), 1-174385 (1989), 3-58787 (1991), 3-164185 (1991), 5-84067 (1993), and 5-192164 (1993)) and the genus Brevibacterium (Japanese Patent Application Laid-Open No. 63-248394 (1988)).